The present invention relates to a process for preparing ethylbenzene using vapor phase alkylation and liquid phase transalkylation. One embodiment of the present invention includes an ethylbenzene process in which a series-arranged or combined vapor phase alkylation/transalkylation reaction zone is retrofitted to have a vapor phase alkylation reactor and a liquid phase transalkylation reactor. Another embodiment of the present invention includes an ethylbenzene process in which a parallel-arranged vapor phase alkylation reactor and vapor phase transalkylation reactor is retrofitted to have a vapor phase alkylation reactor and liquid phase transalkylation reactor. Still another embodiment is an apparatus for the practice of the ethylbenzene process of this invention.
Ethylbenzene is a valuable commodity chemical which is used industrially for the production of styrene monomer, most of which is used to make polystyrene. Ethylbenzene may be produced by a number of different chemical processes. One process which has achieved a significant degree of commercial success is a vapor phase ethylbenzene process in which benzene is alkylated with ethylene in the presence of an alkylation catalyst, such as solid, acidic zeolite catalyst comprised of a solid, crystalline aluminosilicate, i.e., ZSM-5 zeolite. In the first generation vapor phase ethylbenzene process, the reaction takes place in a single reactor having a series-arranged alkylation/transalkylation reaction zone that is maintained under suitable vapor phase alkylation/transalkylation conditions. Ethylbenzene is produced along with polyalkylated byproducts and xylenes byproducts. The polyalkylated byproducts are referred to as “polyethylbenzenes” when used in connection with the alkylation of benzene with ethylene, and such polyethylbenzene include diethylbenzene, triethylbenzene, tetraethylbenzene, and pentaethylbenzene and hexaethylbenzene. Ethylbenzene and a small amount of its co-boilers, such as xylene, are separated from the polyalkylated byproducts. Such ethylbenzene is typically used as styrene monomer feed. The remaining polyalkylated byproducts are recycled to the alkylation/transalkylation reaction zone.
In the second generation vapor phase ethylbenzene process, a parallel-arranged alkylation reactor and transalkylation reactor are used to produce ethylbenzene. In the alkylation reactor, benzene is alkylated with ethylene in the presence of a solid, acidic zeolite catalyst and under suitable vapor phase conditions to form ethylbenzene and polyalkylated byproducts and xylenes byproducts. Ethylbenzene and its co-boilers, such as xylene, are separated from the polyalkylated byproducts and typically used as styrene monomer feed as in the earlier process. The amount of ethylbenzene co-boilers, such as xylenes, is typically lower with such second generation vapor phase ethylbenzene processes as compared to its first generation counterparts. The second generation vapor phase process as differs from the first generation process in that the polyalkylated byproducts, such as polyethylbenzenes, are sent to a separate transalkylation reactor. In the transalkylation reaction, the polyalkylated byproducts is contacted with benzene in the presence of a transalkylation catalyst, such as solid, acidic catalyst comprised of a solid, crystalline aluminosilicate, i.e., ZSM-5 zeolite, to produce additional ethylbenzene and a reduced amount of polyalkylated byproducts.
Examples of such vapor phase ethylbenzene processes are described in U.S. Pat. Nos. 3,751,504 (Keown), 4,547,605 (Kresge) and 4,016,218 (Haag).
Another ethylbenzene process which has achieved significant commercial success is the all liquid phase process for producing ethylbenzene from benzene and ethylene. This all liquid phase process operates at a lower temperature and higher pressure than its vapor phase counterparts, but with often greater ethylbenzene capacity and lower yields of polyalkylated byproducts and xylenes byproducts, as compared to such vapor phase counterparts. For example, U.S. Pat. No. 4,891,458 (Innes) describes the liquid phase synthesis of ethylbenzene with zeolite Beta, whereas U.S. Pat. No. 5,334,795 (Chu) describes the use of MCM-22 in the liquid phase synthesis of ethylbenzene.
Even with the lower byproduct yields in the all liquid phase process for producing ethylbenzene, polyalkylated byproducts and xylenes byproducts are still inherently produced. The polyalkylated byproducts are transalkylated with additional benzene in a separate transalkylation reactor as in the second generation vapor phase processes, to produce additional ethylbenzene and a reduced amount of polyalkylated byproducts, however such transalkylation is conducted under suitable liquid phase transalkylation conditions. The amount of ethylbenzene co-boilers, such as xylenes, is typically lower with such all liquid phase ethylbenzene processes as compared to its first generation and second generation counterparts. Examples of catalysts which have been used in the all liquid phase processes for the alkylation of benzene with ethylene and for the transalkylation of polyalkylated byproducts, such as diethylbenzenes, are listed in U.S. Pat. No. 5,557,024 (Cheng) and include MCM-22, PSH-3, SSZ-25, zeolite X, zeolite Y, zeolite Beta, acid dealuminized mordenite and TEA-mordenite. Transalkylation over a small crystal (<0.5 micron) form of TEA-mordenite is also disclosed in U.S. Pat. No. 6,984,764.
The older, vapor phase ethylbenzene processes may be retrofitted to the newer, all liquid phase processes, in order to obtain the higher capacities and lower yields of the polyalkylated byproducts and xylenes byproduct. However, the cost to retrofit these older, vapor phase ethylbenzene processes to a full liquid phase process, i.e., liquid phase alkylation combined with liquid phase transalkylation, may be high. For example, to retrofit certain first generation, vapor phase ethylbenzene processes (series-arranged alkylation/transalkylation reaction zones in a single reactor) to full liquid phase processes, a new transalkylation reactor must be installed and the single must be converted from combined alkylation and transalkylation service to only alkylation service. Also, high capacity transfer pumps must be installed to maintain gaseous ethylene in the liquid phase with benzene.
Similarly, to retrofit certain second generation vapor phase ethylbenzene process (parallel-arranged alkylation and transalkylation reactors) to a full liquid phase process, for example, both the alkylation and transalkylation reactors must be converted from vapor phase service to liquid phase service. In some retrofits, the size of the reactors must be increased. As in retrofitting the first generation vapor phase processes, high capacity transfer pumps must be installed to maintain gaseous ethylene in the liquid phase with benzene.
The costs to retrofit older, vapor phase ethylbenzene processes to the newer, full liquid phase processes, having higher capacity and lower polyalkylated byproduct yields, has proven to be a significant deterrent. Efforts have been made to revamp older, ethylbenzene processes having a vapor phase alkylation reactors and vapor phase translation reactors, such as in second generation ethylbenzene processes, by converting the vapor phase alkylation reactor to a liquid phase alkylation reactors, while maintaining the transalkylation reactor in the vapor phase.
In U.S. Pat. No. 5,600,048 (Cheng), a continuous process for preparing ethylbenzene using liquid phase alkylation and vapor phase transalkylation is disclosed. The liquid phase alkylation reaction may be catalyzed by an acidic solid oxide, such as MCM-22, MCM-49 and MCM-56. The vapor phase transalkylation may be catalyzed by a medium-pore size zeolite such as ZSM-5. The process may be run continuously with the continuous introduction of fresh benzene feed containing at least 500 wppm of nonbenzene hydrocarbon impurities. The combined ethylbenzene product of these alkylation and transalkylation reactions has very low levels of impurities including xylene, hydrocarbons having 7 or less carbon atoms and hydrocarbons having 9 or more carbon atoms.
U.S. Pat. No. 5,995,642 (Merrill) discloses an alkylation/transalkylation process involving vapor phase alkylation of a benzene feedstock in a multi-stage alkylation zone having a plurality of series connected catalyst beds containing a pentasil aromatic alkylation catalyst, such as silicalite, coupled with intermediate separation and recirculation steps and liquid phase transalkylation over a transalkylation catalyst comprising a molecular sieve having a pore size greater than the pore size of the silicalite. The benzene containing feedstock is supplied to the multi-stage alkylation reaction zone along with a C2-C4 alkylating agent operated under temperature and pressure conditions to maintain the benzene in the gas phase. Alkylated product is recovered from the alkylation zone and supplied to a benzene recovery zone for the separation of the benzene from the alkylation product. Benzene from the benzene recovery zone is recycled to the reaction zone. A higher boiling bottom fraction containing a mixture of monoalkylated and polyalkylated aromatic components is supplied to a secondary separation zone from which a monoalkylated aromatic component, e.g. ethylbenzene, is recovered overhead with a heavier polyalkylated aromatic recovered as a bottom fraction. The bottom fraction may be applied to a tertiary separation zone.
U.S. Pat. No. 6,897,346 (Merrill) discloses a process for the transalkylation of polyalkylated aromatic compounds over a high porosity zeolite-Y molecular sieve having a surface area of no more than 500 m2/g. A feedstock comprising a polyalkylated aromatic component, including polyalkylbenzenes in which the predominant alkyl substituents contain from 2 to 4 carbon atoms, is supplied to a transalkylation reaction zone containing the high porosity zeolite-Y catalyst. Benzene is also supplied to the transalkylation zone, and the reaction zone is operated under temperature and pressure conditions to maintain the polyalkylated aromatic component in the liquid phase and which are effective to cause disproportionation of the polyalkylated aromatic component to arrive a disproportionation product having a reduced polyalkylbenzene content and an enhanced monoalkylbenzene content. An alkylation reaction zone is provided which contains a molecular sieve aromatic alkylation catalyst having an average pore size which is less than the average pore size of the average pore size of the high porosity zeolite-Y. A feedstock comprising benzene in a C2-C4 alkylating agent is supplied to the alkylation reaction zone which is operated under conditions to produce alkylation of the benzene by the alkylating agent in the presence of the molecular sieve alkylation catalyst. The alkylation product from the alkylation reaction zone is supplied to an intermediate recovery zone for the separation and recovery of a monoalkylbenzene, e.g. ethylbenzene, from the alkylation product; together with the recovery of a polyalkylated aromatic component employing a dialkylbenzene, e.g. diethylbenzene. The polyalkylated aromatic component is employed in at least a portion of the feedstream supplied to the transalkylation reactor.
WO 94/13603 (Abichandani et al.) discloses a process for producing ethylbenzene, wherein benzene is alkylated with ethylene in a vapor phase reaction over a catalyst comprising ZSM-5. Diethylbenzene byproduct from the vapor phase alkylation reaction is separated from the ethylbenzene product and reacted with benzene in a liquid phase transalkylation reaction to produce more ethylbenzene. The catalyst for the liquid phase transalkylation reaction may comprise a zeolite, such as zeolite beta. The combined ethylbenzene product from the vapor phase alkylation reaction and from the liquid phase transalkylation reaction has a low xylene impurities level of less than 1000 ppm.
None of these prior art process teaches an ethylbenzene process having a vapor phase alkylation reactor combined with a liquid phase transalkylation reactor, wherein the combined ethylbenzene product from the vapor phase alkylation reaction and from the liquid phase transalkylation reaction has a low xylene impurities level of less than 700 wppm. Therefore, there is a need for such ethylbenzene processes.
According to the present invention, it has now unexpectedly been found that older, vapor phase ethylbenzene process may be successfully retrofitted to an improved process that combines vapor phase alkylation with liquid phase transalkylation, wherein such improved process has higher ethylbenzene capacity and lower yields of polyalkylated byproduct and xylenes byproduct as compared to an all vapor phase ethylbenzene process. In first generation, vapor phase ethylbenzene processes, for example, the single reactor having series-arranged alkylation and transalkylation zones is converted to alkylation service and a liquid phase transalkylation reactor is installed. In second generation, vapor phase ethylbenzene processes, for example, the transalkylation reactor is converted to liquid phase and the vapor phase alkylation reaction is substantially unmodified. In both such examples, the addition of larger transfer pumps is minimized. More importantly, in both such vapor phase ethylbenzene processes, the concomitant benefits of increased ethylbenzene capacity and a lower xylenes byproduct yields may be realized.